A π-Conjugation Extended Viologen as a Two-Electron Storage Anolyte for Total Organic Aqueous Redox Flow Batteries

Extending the conjugation of viologen by a planar thiazolo[5,4-d]thiazole (TTz) framework and functionalizing the pyridinium with hydrophilic ammonium groups yielded a highly water-soluble pi-conjugation extended viologen, 4,4'-(thiazolo[5,4-d]thiazole-2,5-diyl)bis(1-(3-(trimethylammonio)propyl)pyridin-1-ium) tetrachloride, [(NPr)(2)TTz]Cl-4, as a novel two-electron storage anolyte for aqueous organic redox flow battery (AORFB) applications. Its physical and electrochemical properties were systematically investigated. Paired with 4-trimethylammonium-TEMPO (N-Me-TEMPO) as catholyte, [(NPr)(2)TTz]Cl-4 enables a 1.44 V AORFB with a theoretical energy density of 53.7 Wh L-1. A demonstrated [(NPr)(2)TTz]Cl-4/N-Me-TEMPO AORFB delivered an energy efficiency of 70% and 99.97% capacity retention per cycle.